{"id":4898,"date":"2013-04-29T14:46:14","date_gmt":"2013-04-29T12:46:14","guid":{"rendered":"https:\/\/ingmarverheij.com\/?p=4898"},"modified":"2013-06-15T13:47:14","modified_gmt":"2013-06-15T11:47:14","slug":"demystifying-the-atlantis-ilio-replication-host-part-2","status":"publish","type":"post","link":"https:\/\/ingmarverheij.com\/en\/demystifying-the-atlantis-ilio-replication-host-part-2\/","title":{"rendered":"Demystifying the Atlantis ILIO replication host – part 2"},"content":{"rendered":"

In the first part<\/a> of this article I\u2019ve explained what Atlantis ILIO products and components there are, explained what type of Session Hosts there are and how the Replication Host works. In this part I will dive a bit deeper in the boot process and the how the hosts are managed.<\/p>\n

\"Atlantis<\/a><\/p>\n

<\/p>\n

Boot process<\/font><\/h2>\n

So what happens is a Session- or Replication Host starts-up? What has happened to make the datastore available for the virtual machines?<\/p>\n

The boot process of the hosts consists of booting up the operating system (Ubuntu 8.10) , boot to Multi-user mode (runlevel 2<\/a>) and finally run the \/etc\/rc.local<\/em> script. Each Atlantis ILIO host has a specific boot process, yet they share a great similarity.

<\/p>\n

<\/font><\/h5>\n
Actions<\/a><\/font><\/h5>\n

During the execution of the start-up script \/etc\/rc.local <\/em>a number of actions are executed. In table below all actions are listed and explained.<\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n
Action<\/strong><\/td>\nDescription<\/strong><\/td>\n<\/tr>\n
Network Optimization<\/td>\nFor each network interface [ethx<\/em>] the TCP\/IP stack is optimized for IP storage \/ maximum throughput by enabling TCP Segment Offload<\/a> (TSO), increasing the ring receive (RX) buffer to 4096, increasing the ring send (tx) buffer to 4096 and setting the transmit queue length (txqueuelen) to 300000. <\/td>\n<\/tr>\n
Set RAID minimum network speed<\/td>\nConfigures the minimum RAID speed equal to the maximum speed of the network (2Gbit). <\/p>\n

This command is only issued on the \u201cAtlantis ILIO Persistent VDI \u2013 Session Host\u201d  (functiontype DLN) because that\u2019s the only machine using linux-raid<\/a> (called \u201cAtlantis FastReplication\u201d) as explained in part 1<\/a> of this article.
<\/em><\/td>\n<\/tr>\n

Reconstruct ILIO cache<\/td>\nThe memory cache of ILIO (used for inline de-duplication and assembling small random blocks to large sequential blocks) is reconstructed to ensure a consistent state.  <\/p>\n

It ensures that the VMs (in the instance of a disk-based Session host or a Replication Host) are read in from the permanent environment, recreated and re-populated. <\/td>\n<\/tr>\n

Synchronize file systems<\/td>\nThis is essentially a checkpoint. The ILIO host is doing a "we\u2019re all good – \u201cstart here\u201d. Effectively creating a synchronization point, this is the point in time you rollback to when non-persistent machines are rebooted.  <\/td>\n<\/tr>\n
Start ILIO Center Agent<\/td>\nThe ibert daemon is started (also known as the ILIO Center Agent) to enable ILIO Center to manage and monitor the host. <\/td>\n<\/tr>\n
Mount nfsd volume<\/td>\nMounts the the nfsd filesystem<\/a> to \/proc\/fs\/nfsd<\/em> which is required to provide access to the Linux NFS server. The file system consists of a single directory which contains a number of files. These files are actually gateways into the NFS server. <\/td>\n<\/tr>\n
Start NFS server<\/td>\nStarts the NFS server daemon on the host to enable remote access to the \/exports\/ILIO_VirtualDesktops<\/em> mount point and all other mount points stored in the \/etc\/exports<\/em> file. <\/td>\n<\/tr>\n
Configure disk<\/td>\nFor each disk [\/dev\/hdx] parameters are set to optimizes the caching, buffering, etc. to ensure a proper writing. *more info needed*<\/strong> <\/td>\n<\/tr>\n
Restore SnapClone<\/td>\nRestores a snapshot from folder \/mnt\/images<\/em> (linked to \/dev\/sdb1)<\/em> to \/dev\/(z)ram0<\/em> (RAM disk) using the PartClone<\/a> tool. <\/p>\n

The snapshot is restored to ensure that all files that where stored on the host, like the configuration (.vmx) and virtual hard disk (.vmdk) are available. Once the files are available the hypervisor host can boot the virtual machine. If no snapshot is restored, and the host is restarted, an empty datastore is presented to the hypervisor host. <\/p>\n

This command is only issued on the \u201cAtlantis ILIO Diskless VDI \u2013 Session Host\u201d. Normally all files are stored on the RAM disk, which is volatile. Unlike the \u201cAtlantis ILIO Persistent VDI \u2013 Session Host\u201d a constant replication with a SAN is not wanted for two reasons 1) files should only persist when issued by the administrator 2) the goal of the diskless solution is to prevent ALL IOPS to the SAN
<\/em><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

A special thanks to Andrew Wood<\/a> (Solutions Engineer @ Atlantis Computing) for providing additional information.<\/em><\/p>\n


<\/p>\n

<\/em><\/p>\n

Similarities and differences<\/font><\/h5>\n

As explained in part 1<\/a> of this article there are different Atlantis ILIO hosts. To keep things simple we\u2019re focusing on the \u201cAtlantis ILIO Diskless VDI\u201d and the \u201cAtlantis ILIO Persistent VDI\u201d product.<\/p>\n

\"Atlantis<\/a>
<\/p>\n

<\/font><\/font><\/h6>\n
Persistent \u2013 Replication Host <\/font>versus Diskless \u2013 Session Host<\/font><\/font><\/h6>\n

The start-up process of the \u201cAtlantis ILIO Persistent <\/strong>VDI \u2013 Replication Host\u201d is equal to the  \u201dAtlantis ILIO Diskless <\/strong>VDI \u2013 Session Host\u201d , with the exception of the SnapClone restore. As already concluded in the first part<\/a> the Replication Host is basically an improved Session Host, just like a session host it receives files and via a NFS mount-point and stores it on the SAN. <\/p>\n

 <\/h6>\n
Atlantis SnapClone<\/h6>\n

Because the \u201cAtlantis ILIO Diskless VDI \u2013 Session Host\u201d only stores files in a volatile RAM disk, all changes are lost after a power-cycle. To retain the virtual machines configuration (.vmx) and hard disk (.vmdk) file an \u201cAtlantis SnapClone\u201d is created by the administrator once the virtual machines are created, the snapshot is restored from disk [\/dev\/sdb1<\/em>] to memory [\/dev\/(z)ram0<\/em>].<\/p>\n

 <\/p>\n

Persistent – Replication Host versus <\/strong>Persistent – Session Host<\/h6>\n

The Session- and Replication Host of the \u201cAtlantis ILIO Persistent VDI\u201d product again share a lot of similarities but have two differences.<\/p>\n

    \n
  1. On the Session Host a minimum speed is for RAID is configured. This command is only issued on the Session Host  because that machine uses a linux-raid<\/a> (called \u201cAtlantis FastReplication\u201d), as explained in part 1<\/a> of this article. <\/li>\n
  2. On the Session Host the NFS Server is started after<\/strong> the disks are configured, where on the other hosts it is started earlier in the process. It would make more sense to me that the NFS server is started last on all <\/strong>hosts, but somehow Atlantis Computing choose differently. <\/li>\n<\/ol>\n

     <\/p>\n

     <\/p>\n

    <\/strong><\/p>\n

    Host management <\/font><\/h2>\n

    With the introduction of Atlantis ILIO Diskless VDI centralized management of the Sessions Hosts became mandatory, primarily to create\/restore SnapClones. Atlantis ILIO Center<\/a> fulfilled this role and introduced health monitoring and reporting. Nonetheless the  Session Hosts could operate independently, they where isolated entities. <\/p>\n

     <\/p>\n

    Session Cluster<\/font><\/h5>\n

    With the introduction of the Replication Host the Sessions Host are no longer isolated entities, they have a mapping with each other. A Session Cluster maps a Replication Host to a number of Session Hosts. One Atlantis ILIO Center can manage multiple Session Clusters (if needed).<\/p>\n

    \"Atlantis<\/a><\/p>\n

    A Session Cluster increases complexity, all components needs to be aware of the mapping between the hosts:<\/p>\n